It is known that for the use of a laser tube it is necessary to proceed first to a pre-ionization of the laser medium and then excite the laser by a high-performance pulse which is fed into the pre-ionized plasma. This is done in a time interval during which a population inversion can be reached, i. e. a molecule displacement to an excited state in which the number of excited molecules is greater than that of molecules at lower energy level. The laser emission is provoked when the excited molecules fall back to their low energy level.
In the case of gas lasers with transverse excitation, e.g. excimer lasers, x-rays or ultraviolet rays are often used to pre-ionize the laser medium. However, the devices for pre-ionization by means of x-rays or UV-rays are expensive and necessitate a large place for the installation. Moreover, in the case of metal vapour lasers it is not possible to use a pre-ionization by means of x-rays of UV-rays because of the high discharge frequency (1 to 100 kHz), the low discharge current (100 to 1000 A) and the longitudinal type of excitation, i.e. the application of an electric field parallel to the output direction of the laser beam. With lasers of this type preionization is made by using part of the electric discharge. During the pre-ionization the energy yielded by an oscillating circuit is partly stored in a capacitor, which is discharged into the laser tube when the discharge in the latter begins. However, the discharge of the storage capacitor cannot be controlled, and so it can happen that at the moment of discharge the plasma is not sufficiently pre-ionized. In this case the impedance of the laser tube is not favorable for optimization of the discharge parameters as, for example, the electric field (optimum between 100 and 200 V/cm) or the energy density fed into the plasma (optimum 1 mJ/cm.sup.3).
Moreover, if a high laser performance is to be reached with metal vapour lasers, it is necessary to increase the volume of the laser tube. It can then be observed that the output of laser energy per volume unit of the plasma decreases. In that case the following phenomena are observed:
(a) the volume recombination in the plasma takes longer time and it becomes necessary to use lower discharge frequencies. This brings about a loss of the residual pre-ionization which normally results from the repetition frequency. Therefore, the time until the discharge is built-up in the tube becomes increasingly longer;
(b) the excitation circuit has to yield a performance which is proportional to the volume of the tube. This is incompatible with the reduction of the discharge frequency;
(c) if the dimensions of the laser tube are incresed the tube inductance increases and the resistnce decreases. As a result of this the current rise time increases and is then generally longer than the maximal time interval of about 50 ns during which the population inversion can be maintained.